Printed electronics (PE) technology harnesses the existing manufacturing capabilities of the graphics industry to produce circuitries cheaply and quickly, and has garnered remarkable attention in the last decade. This technology is transforming the electronics industry by replacing traditional costly methods of fabricating electronic components, devices or even systems. Increasingly, printed thin-film transistors, conductors, inductors and capacitors are being integrating with electronics devices to develop novel systems, such as thin-film energy harvesting/storage system, smart labels, radio frequency identification (RFID) tags and memory devices. A world full of flexible, wearable, even stretchable devices using printing technology is foreseeable in the near future.
Many demonstrations of paper electronics have been made recently; however, existing applications of paper electronics involve the use of plastic-covered paper substrates, photopaper lamination of a plastic film (electronics paper tickets) or the gluing of electronics components, or silicon chips onto a porous substrate. These substrates have better chemical and physical properties than regular cellulose paper, but are generally more than 10 times as expensive. Fabricating a highly conductive circuit on a porous substrate is challenging as the porous substrate typically has high roughness, and cellulose fiber forms a highly porous structure that tends to absorb functional materials (e.g. metal nanomaterials, carbon nanotubes) instead of leaving them on the surface. This prevents conductive materials in the ink from contacting each other, making it impossible to form a highly conductive layer even after sintering, which leads to relatively poor performance in paper-based electronics. Additionally, the capillary effect of the porous substrate also causes a significant loss of resolution when printing with solvent-based ink.
Furthermore, the thickness of the conductor is crucial to many electronics applications. For the same conductor, a thicker layer means a smaller sheet resistance, and thus the thickness usually determines the maximum current the circuits can handle. In the electronics industry, a standard printed circuit board with a 35 μm thick copper layer is adopted for most devices. IoT requires a large number of RF devices to communicate with each other and harvest wireless energy for power. Typically, if the working frequency is higher than 1 MHz, then we need to consider the skin effect, i.e. the antenna conductor has to reach a certain thickness for optimum performance. For example, a copper antenna operating at 13.56 MHz has a skin effect depth of 17.7 μm which means the thickness of the printed antenna has to be at least approximately 17.7 μm for best performance. However, direct printing of conductive materials via a roll-to-roll compatible digital printing process cannot reach this level, which greatly limits its application in both RF devices and regular printed circuits.
All of these various obstacles cause traditional printed electronics to suffer in performance and resolution. Thus, it is important to find a solution to these issues to fully utilize the low-cost, environmental-friendly properties of cellulose paper and other porous substrate for printed electronic technologies.
Electroless metal deposition (ELD), which relies on an autocatalytic redox reaction to deposit various metals on a catalyst-preloaded substrate, offers a low-cost yet convincing solution to the thickness issue. Printed circuits fabricated using ELD have been demonstrated on various substrates such as PET, PI, photopaper, and even yarns. The thickness of the deposited metal layer can be finely tuned by the deposition time, but new challenges concerning adhesion and diffusion appear when thickness is increased. Untreated flexible substrates struggle with capturing catalyst moieties due to lack of binding sites, and simple physical absorption cannot prevent peeling of the deposited metal, especially if the thickness of the deposited metal exceeds 5 μm.
For porous substrate like cellulose paper, the loosely deposited metal particles tend to migrate out of the printed edge, resulting in a severe loss of resolution. As deposition time increases to achieve a thicker metal layer, more and more traces in the circuit will form connections with one another and form short circuits. Surface modification techniques such as UV-oxygen plasma, surface silanization, polyelectrolyte multilayer (PEM), and polymer grafting have been reported to enhance the adhesion between the electroless deposited metal layer and substrate. However, most of these techniques are currently far away from being a scalable cost-effective production method, due to their complexity and/or environmental impact and harsh experimental requirements. Thus, there is a need to develop a simple, low-cost and efficient surface modification method for all kinds of porous substrates to fabricate high resolution thick copper (>20 um) paper-based electronics with strong metal-fiber bonding.
Poly (4-vinylpyridine) (P4VP) has been used for surface modification purposes to uptake silver ions due to its strong chelating ability with transitional metal ions. As a reactive monomer, 4-vinylpyridine has been used to modify substrates via in-situ polymerization triggered by UV and/or plasma. Such cross-linked molecules form covalent bonds with the pretreated substrate, achieving good adhesion. However, a low film production rate and high equipment demands make this method not cost-effective and unsuitable for coating cellulose paper. P4VP molecules can be directly coated onto the substrate by physical absorption, but the poor adhesion will result in serious delamination of the electroless deposited metal.
Generally, manufacturing highly conductive circuits in a short time period using electroless metal deposition remains a challenge. The electroless metal deposition requires a relatively long time to make the circuit highly conductive because metal growth always happens on the surface for traditional methods. Meanwhile, it is impossible to manufacture multilayer circuits at one time without a drill. Such obstacles limit the application of electroless metal deposition in the manufacturing of printed electronics, especially for a roll-to-roll process.